1. Field of the Invention
This invention relates to an optical disc. In addition, this invention relates to an apparatus for recording and reproducing information on and from an optical disc. Furthermore, this invention relates to a method of recording and reproducing information on and from an optical disc.
2. Description of the Related Art
Optical discs are of a read only type (a playback only type), a recordable type (a write once type), and a rewritable type. A CD (Compact Disc), a VCD (Video CD), and a DVD (Digital Versatile Disc) are optical discs of the read only type. A CD-R and a DVD-R are optical discs of the recordable type. A CD-RW, a DVD-RAM, and a DVD-RW are optical discs of the rewritable type.
Optical discs of the rewritable type have thin recording films which are reversibly changed between two or more different states in accordance with conditions of laser beams applied thereto. Rewritable optical discs include magneto-optical discs and phase change discs.
In the case of a phase change optical disc, while a recording film is scanned by a laser beam, the recording film is reversibly changed between an amorphous state and a crystalline state by varying conditions of the laser beam in response to a signal to be recorded. Thus, the signal is recorded on the recording film as a pattern of amorphous portions and crystalline portions of the recording film. The signal is reproduced from the phase change optical disc as follows. The surface of an amorphous portion of the disc and the surface of a crystalline portion thereof are different in reflectivity with respect to a laser beam. While the phase change optical disc is scanned by a laser beam, a change in reflectivity of the disc surface with respect to the laser beam is optically detected so that the signal is reproduced from the disc.
The phase change optical disc is similar to a read only optical disc and a recordable optical disc in the point that the signal reproduction is implemented by detecting a change in the disc surface reflectivity with respect to a laser beam. The signal overwriting on the phase change optical disc can be performed by use of only one laser beam when the laser power is modulated between an erasing level and a recording level. Therefore, the structure of a drive device for the phase change optical disc can be simple.
A PWM (pulse width modulation) system is used to record a signal on a rewritable optical disc at a high density. According to the PWM system, the positions of the front and rear edges of every recording mark on the disc correspond to “1” in a digital signal.
Conditions of a rewritable optical disc, such as the composition of material for the disc, an additive to the material, and a film thickness in the disc, depend on the maker which has produced the disc. Accordingly, an optimal recording power of a laser beam applied to a rewritable optical disc, an optimal erasing power of the laser beam, a desired width of a front end pulse in a pulse group, a desired width of intermediate multiple pulses in the pulse group, and a desired width of a rear end pulse in the pulse group vary from disc maker to disc maker. In general, information about an optimal recording power, an optimal erasing power of the laser beam, and desired pulse widths which vary from disc maker to disc maker is recorded on each rewritable optical disc as pre-pits.
In the PWM system, the width of every recording mark represents information. Thus, a desirable shape of the recording mark is free from distortion. Specifically, it is desirable that the shapes of the front and rear halves of the recording mark are symmetrical with each other. During the PWM-based recording of a signal on the disc, the disc is exposed to a laser beam while being rotated and moved relative thereto. In addition, the intensity of the laser beam is changed between strong and weak levels in response to the signal to be recorded. Recording marks are formed on portions of the disc which are exposed to and heated by the stronger laser beam. Regarding every recording mark, the heat accumulation effect causes the stronger-beam-application ending point on the disc to be higher in temperature than the stronger-beam-application starting point on the disc. As a result, the rear end of the recording mark is wider than the front end thereof. Thus, the shape of the recording mark is distorted.
A known drive apparatus for a rewritable optical disc records a signal on the disc by use of a laser beam while driving and rotating the disc relative to the laser beam at a constant linear velocity which can be selected from a predetermined normal velocity and a predetermined high velocity. The normal velocity is equal to, for example, 3.49 m/s. The high velocity is equal to, for example, twice or four times the normal velocity.
Since the recording of a signal on a rewritable optical disc by a laser beam is based on heating, recording conditions change in accordance with the velocity (the speed) at which the disc is driven and rotated relative to the laser beam. Accordingly, an optimal recording power of the laser beam, an optimal erasing power of the laser beam, a desired width of a front end pulse in a pulse group, a desired width of intermediate multiple pulses in the pulse group, and a desired width of a rear end pulse in the pulse group depend on the drive speed of the disc relative to the laser beam.
A desired width of a front end pulse in a pulse group, a desired width of intermediate multiple pulses in the pulse group, and a desired width of a rear end pulse in the pulse group are time information referred to as strategy. It is known that information about an optimal recording power of a laser beam, information about an optimal erasing power of the laser beam, and strategy for a normal disc drive speed (a normal disc scanning speed) are recorded on a rewritable optical disc as pre-pits. On the other hand, it is not known to record laser-power information and strategy for a high disc drive speed (a high disc scanning speed) on a rewritable optical disc as pre-pits.
In the case of a rewritable optical disc having pre-pits representing only laser-power information and strategy for a normal disc scanning speed, it takes a long time to find optimal recording conditions for a high disc scanning speed and then start the recording of a signal on the disc under the optimal recording conditions.
U.S. Pat. No. 6,404,713 B1 corresponding to Japanese patent application publication number P2001-209940A discloses a first apparatus for recording and reproducing an information signal on and from an optical disc. The first apparatus includes a memory. The information signal is written into the memory. The information signal is read out from the memory. An optical head generates a laser beam in response to the readout information signal, and applies the laser beam to the optical disc to record the readout information signal on the optical disc. A test signal is recorded on a position of the optical disc near a recording position thereof via the optical head during the writing of the information signal into the memory. The test signal is reproduced from the optical disc. The reproduced test signal is evaluated to generate an evaluation result. An intensity of the laser beam is optimized in response to the evaluation result.
U.S. Pat. No. 6,404,713 B1 also discloses a second apparatus for recording and reproducing an information signal on and from an optical disc. The second apparatus includes a memory. The information signal is written into the memory. The information signal is read out from the memory. An optical head generates a laser beam in response to the readout information signal, and applies the laser beam to the optical disc to record the readout information signal on the optical disc. A power of the laser beam is changed among a plurality of different levels. The laser beam is measured to generate measurement result values during the change of the power of the laser beam among the plurality of the different levels. An intensity of the laser beam is optimized in response to the measurement result values.